1. Field of the Invention
The invention relates to control of a continuously variable transmission, and particularly to a technology that enables, when the vehicle is traveling on an uphill road, appropriate switching of the shift control mode between a mode for securing a sufficient climbing performance of the vehicle and a mode for ensuring that the belt of the continuously variable transmission can be returned to the position corresponding to the initial speed ratio for starting the vehicle from a standstill before the vehicle comes to a stop.
2. Description of the Related Art
Belt-type CVTs (Continuously Variable Transmissions) have been used as vehicle transmissions. A belt-type CVT is constituted of a drive pulley (primary pulley) and a driven pulley (secondary pulley) each having a belt groove having a V-shaped cross section and a belt wound around the drive pulley and the driven pulley. In operation, the width of the groove of one pulley is increased while the width of the groove of the other pulley is reduced, whereby the diameter of the belt loop at each pulley (pitch diameter) continuously changes. This is how the speed ratio of the belt-type CVT changes continuously.
In a vehicle incorporating such a belt-type CVT, the width of the groove of the drive pulley and the width of the groove of the driven pulley are changed using hydraulic actuators so as to achieve a target speed ratio set in accordance with the vehicle speed and the accelerator operation amount.
A technology is known which corrects the target speed ratio of a belt-type CVT based on the running resistance against the vehicle. For example, Japanese Patent Application Publication 2004-162799 (JP-A-2004-162799) describes a CVT shift control apparatus that enables accelerating the vehicle just as intended by the driver upon kick-down acceleration regardless of variation of the running resistance (e.g., road gradient). This CVT shift control apparatus includes operation state detecting means for detecting the operation state of the vehicle including the vehicle speed and the accelerator operation amount, speed ratio setting means for setting the target speed ratio in accordance with the vehicle speed and the accelerator operation amount, and controlling means for controlling the speed ratio of the CVT based on the set speed ratio. Further, the CVT shift control apparatus includes acceleration requirement determining means for determining the degree of acceleration requirement, acceleration shift characteristic setting means for setting the downshift characteristic and the upshift characteristic based on the degree of acceleration requirement, respectively, acceleration controlling means for, when the degree of acceleration requirement is higher than a predetermined reference value, shifting the CVT down to a target speed ratio that is lower than the target speed ratio set by the speed ratio setting means based on the downshift characteristic and then shifting the CVT up based on the upshift characteristic; running resistance detecting means for detecting the running resistance against the vehicle; and speed ratio correcting means for, when the running resistance is larger than a predetermined value, correcting the target speed ratio for downshift based on the magnitude of the running resistance.
Japanese Patent Application Publication No. 2004-162799 describes a shift control apparatus that brings the rising and dropping of the acceleration of the vehicle into balance by suppressing an excessive increase in the engine speed and a decrease in the acceleration of the vehicle when accelerating the vehicle, so that the vehicle can be reliably accelerated just as intended by the driver. This shift control apparatus corrects the target speed ratio for downshift according to the magnitude of the running resistance when the running resistance is higher than a certain value. According to this shift control apparatus, therefore, the driver can always feel the vehicle being accelerated just as he or she intends even if the running resistance (e.g., road gradient) changes, and therefore the drivability of the vehicle improves significantly.
Meanwhile, when a vehicle is braked and stopped on an uphill road, the stopping distance (stopping duration) of the vehicle tends to be relatively short, and in such a case, therefore, the belt of the CVT may fail to return to the position corresponding to the initial speed ratio for starting the vehicle from a standstill before the vehicle comes to a stop. If the belt is not at the initial speed ratio position when starting the vehicle from a standstill, the acceleration performance of the vehicle deteriorates. Note that the capability to return the belt to the initial speed ratio position when the vehicle is being stopped will hereinafter be referred to as “the returnability of the belt” or the like.
In view of the above, it is necessary to switch the shift control mode between a mode for achieving a climbing performance required by the driver, which is determined from the vehicle speed and the accelerator operation amount, and a mode for achieving a sufficient returnability of the belt at the time of stopping the vehicle.
However, the shift control apparatus described in JP-A-2004-162799 does not take into consideration the returnability of the belt at the time of stopping the vehicle, and therefore the above issue can not be solved.